1. Field of the Invention
The present invention relates to a device to form a continuous chain in multi-needle sewing machines for making an ornamental sticth without a cover stitch, of the type comprising: at least two sewing needles carrying respective needle threads and operating through a needle plate upon command of a needle bar operable with a reciprocating motion; a lower looper alternately movable under the needle plate to interlace a corresponding interconnecting thread with said needle threads; a feed unit for the needle threads arranged to feed the needle threads according to a predetermined tensioning towards said needles; an upper thread take-up fastened to the top of the needle bar and operatively engaging the needle threads in order to take up a predetermined amount of thread from the feed unit at every cycle of the needle bar reciprocating movement; means intervening on at least one of said needle threads to release the predetermined tensioning of same.
In the embodiment herein described, the invention aims at executing a continuous chain by means of sewing machines adapted to carry out seams of the type marked by No. 406 in the related U.S. Federal Catalogue No. 751. Obviously, the innovatory principles proposed by the invention can also apply for the purpose of enabling or facilitating the execution of a chain with other types of seam.
2. Prior Art
It is known that when seams are to be made the possibility of an idle operation of the sewing machine is generally required, so that, even in the absence of a workpiece under the presser foot, the cooperation between the different sewing instrumentalities such as needles and loopers may at all events cause a correct interlooping of the sewing threads, giving thereby origin to a continuous chain adapted to be gradually moved away from the working area by means of the feed dogs, without the occurence of undesired entanglements of the threads or other inconveniences.
It is pointed out, however, that in sewing machines provided with two or more needles and designed to execute ornamental stitch seams without cover stitches, as well as other types of multi-needle seams, the chain formation during the idle working steps is very difficult and complicated. In fact, the chain formed with a multi-needle seam, due to its inherent nature, has a very limited structural consistency that does not enable the feed dogs operating under the presser foot to drag it along in a correct manner. As a result, during the idle working steps undesired entanglements and/or breakages of the sewing threads as well as of the needles can easily occur.
In an attempt to solve these problems, different types of devices have been envisaged which are adapted to enable the formation of a continuous chain in multi-needle sewing machines making ornamental stitch seams without cover stitches.
In some of the above devices, such as the one described in French patent issued under No. 2648162, in order to restrain the possibility of thread entanglements in the sewing area, the chain is forced to pass through a narrow slit defined in the needle plate close to the sewing area. Such a slit is such shaped that it exerts a holding action on the chain, so that sliding of the chain itself takes place with a predetermined friction. It is to be pointed out, however, that the devices of this type have proved to be unsatisfactory in terms of operating reliability, in that the efficiency of same is greatly affected by many variable parameters such as for example the material and sizes of the threads used, their degree of humidity and still other values.
In another technical solution described in the Italian Patent No. 1077525 it is essentially provided that in the absence of a workpiece under the presser foot, the selective intervention of one or more actuators acting on the plates of the thread tension members usually provided in the sewing machine is caused, in order to release or eliminate tensioning of one or more threads associated with the needles. In particular, when an ornamental stitch without a cover stitch is to be executed with a two-needle machine, provision is made for the thread of the outer needle, that is the one designed to operate closer to the fabric edge, to be slackened. Slackening or tension release on the needle thread causes the latter to be obliquely deviated, instead of keeping an aligned positioning in the extension of the corresponding needle, when the lower looper, on carrying out the usual reciprocating transverse movement, is inserted into the loops formed by the needle threads when the needles are carrying out their upward stroke. The needle thread deviation from its normal position causes said thread to be newly interlooped by the corresponding needle while the subsequent sewing stitch is being made. Under this circumstance, the chain is made in the form of a cord and, due to its structural features in terms of consistency and compactness, the correct dragging along of said chain by the feed dogs is promoted.
However, even with the use of devices based on the above principle, many problems arise as regards efficiency and operating reliability and, as a result, frequent breakages of the chain while it is formed and/or entanglements of the sewing threads occur.
It has been found that said problems originate essentially from the fact that the needle thread tensioning reduction or release is advantageous as regards the possibility of deviating the thread orientation while the needles are moving upwardly, but it is as much disadvantageous during all the other stitch formation steps where for the engagement of the individual threads by the sewing members the threads are required to be carefully tensioned so that they may keep a positioning as precise as possible.
Therefore it has been found necessary to release the needle thread tensioning during the needle raising step alone, tensioning being restored to the usual value during all the other stitch formation steps. On the other hand, the possibility of meeting the above requirement by means of the usual devices directly intervening on the thread extension members is unthinkable, in that the releasing step and the step for restoring the thrust action between the plates arranged therein would have to take place at very high rates, which is inconsistent with the mechanical features of such members.